Demineralisation of tap water, produced from Surface water (lake), by a membrane distillation process has been investigated. The temperature caused the decomposition of bicarbonates presented in the feeding water and the formation of alkaline scaling oil the membrane Surface. In the present work the influence of various operating parameters such as the feed temperature and the hydrodynamics conditions oil the formation of the deposit layer, mainly calcium carbonate, Was Studied. The deposits were analysed by scanning electron microscopy (SEM) coupled with energy dispersion spectrometry (EDS). For the low flow rate a deposit layer was more compact than that formed for higher flow rates. The morphology of the formed deposit has a significant Influence oil the permeate flux decline., and a higher flux was found for the less compact deposit layer, The alkaline scaling was retarded for the feed temperature below 353 K and the flow rates above 0.6 m/s.